The present invention relates generally to an exposure apparatus, and more particularly to an exposure apparatus used to expose objects, such as single crystal substrates for semiconductor wafers and glass plates for liquid crystal displays (“LCD”).
A reduction projection exposure apparatus has been conventionally employed that uses a projection optical system to project or transfer a circuit pattern on a reticle (or a mask) onto a wafer, etc., in order to manufacture fine semiconductor devices such as a semiconductor memory or a logic circuit in photolithography technology.
Recent demands for smaller and thinner profile electronic devices have increasingly called for finer semiconductor devices to be mounted onto these electronic devices. The exposure apparatus is required to have such high optical performance as the critical dimension (or resolution) on a wafer surface of 0.2 μm or, preferably, 0.1 μm.
The semiconductor industry has recently shifted its production to a highly value-added system chip that mixes a wide variety of patterns the reticles needing plural types of patterns. Reticle patterns include an adjacent and periodic line and space (L & S) pattern, a line of contact holes that are adjacent and periodic (i.e., arranged at the same interval as the holes diameters), isolated contact holes that are non-adjacent and isolated, other isolated patterns, etc. A transfer of a pattern with high resolution requires a selection of optimal exposure conditions in accordance with these kinds of patterns.
In order to handle exposure processes of various featured patterns, more specifically, to set an exposure condition suitable for each exposure process, an exposure apparatus has been proposed (for example, in Japanese Patent Application, Publication No. 5-299321) that can change a numerical aperture (“NA”) in the projection optical system, an illumination condition, such as a coherence factor σ (i.e., a ratio of an illumination optical system's NA to a projection optical system's NA), and a σ distribution in the illumination area (for a so-called modified illumination, such as an oblique incidence illumination, a multi-pole illumination, and an off-axis illumination).
However, when the projection optical system's NA, the illumination optical system's NA and an effective light source shape are changed independently, the illumination light from the illumination optical system can be larger than the projection optical system's NA and disadvantageously shielded by the projection optical system. As a result, the imaging performance deteriorates and the light intensity becomes uneven. In particular, the recently frequently used modified illumination among the resolution-enhanced technology (“RET”) has a large σ value, thus this problem is likely to happen.
The exposure apparatus proposed in the above reference includes a means for alarming an error or prohibiting an exposure action when an operator's setting causes the 0-th order light in illumination light, that passes through an illumination stop and the reticle, not to pass through the projection optical system. Thus, this reference arduously requires the operator to avoid this problem at the time of the setting.
In addition, as the off-axis telecentricity is adjusted for corrections of on-axis and off-axis telecentricity, the outline of the σ distribution decenters, the projection optical system can similarly shield the illumination light, and the imaging performance deteriorates. It is therefore important that the illumination light (in particular, the 0-th order light) enters the projection optical system in view of its NA even when the σ distribution decenters.